The use of acoustic (e.g., ultrasonic) measurement systems in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications is well known. In known systems an acoustic sensor, typically with a substantially homogenous piezo-ceramic structure on board, operates in a pulse-echo mode in which it is utilized to both send and receive a pressure pulse in the drilling fluid (also referred to herein as drilling mud). In use, an electrical drive voltage (e.g., a square wave pulse) is applied to the transducer, which vibrates the surface thereof and launches a pressure pulse into the drilling fluid. A portion of the ultrasonic energy is typically reflected at the drilling fluid/borehole wall interface back to the transducer, which induces an electrical response therein. Various characteristics of the borehole, such as borehole diameter and measurement eccentricity and drilling fluid properties, may be inferred utilizing such ultrasonic measurements. For example, U.S. Pat. No. 4,665,511 to Rodney et al., discloses a System for Acoustic Caliper Measurements using ultrasonic measurements in a borehole, while U.S. Pat. No. 4,571,693 to Birchak et al., discloses an Acoustic Device for Measuring Fluid Properties that is said to be useful in downhole drilling applications. Numerous other prior art acoustic measurement systems are available in the prior art, including for example, U.S. Pat. RE 34,975 to Orban et al., U.S. Pat. No. 5,469,736 to Moake, U.S. Pat. No. 5,486,695 to Schultz et al., and U.S. Pat. No. 6,213,250 to Wisniewski et al.
While prior art acoustic sensors have been used in various downhole applications (as described in the previously cited U.S. patents), their use, particularly in logging while drilling (LWD) and measurement while drilling (MWD) applications, tends to be limited by various factors. As used in the art, there is not always a clear distinction between the terms LWD and MWD, however, MWD typically refers to measurements taken for the purpose of drilling the well (e.g., navigation) whereas LWD typically refers to measurement taken for the purpose of analysis of the formation and surrounding borehole conditions. Nevertheless, these terms are hereafter used synonymously and interchangeably.
Most prior art acoustic measurement systems encounter serious problems that result directly from the exceptional demands of the drilling environment. Acoustic sensors used downhole must typically withstand temperatures ranging up to about 200 degrees C. and pressures ranging up to about 25,000 psi. In many prior art systems, expansion and contraction caused by changing temperatures is known, for example, to cause delamination of impedance matching layers and/or backing layers from surfaces of the transducer element. Further, the acoustic sensors are subject to various (often severe) mechanical forces, including shocks and vibrations up to 650 G per millisecond. Mechanical abrasion from cuttings in the drilling fluid, and direct hits on the sensor face (e.g., from drill string collisions with the borehole wall) have been known to damage or even fracture a piezo-ceramic element in the transducer. A desirable acoustic sensor must not only survive the above conditions but also function in a substantially stable manner for up to several days (time of a typical drilling operation) while exposed thereto.
Existing acoustic measurement systems also tend to be limited in downhole environments by transducer ringing and a relatively poor signal to noise ratio (as compared to, for example, transducers used in other applications). As such, typical prior art acoustic sensors are typically imprecise at measuring distances outside of a relatively narrow measurement range. At relatively small distances (e.g., less than about one centimetre) acoustic measurements tend to be limited by residual transducer ringing and other near field limitations related to the geometry of the transducer. At relatively larger distances (e.g., greater than about 8 centimetres) acoustic measurements tend to be limited by a reduced signal to noise ratio, for example, related to the transmitted signal amplitude and the receiver sensitivity required to overcome drilling mud attenuation and formation/mud impedance contrast effects.
Therefore, there exists a need for an improved acoustic sensor for downhole applications. While the above described limitations are often associated with the transducer element (i.e., the piezo-ceramic element in prior art downhole devices), and thus represent a need for improved transducers for down hole applications, there also exists a need for improved impedance matching layers and backing layers (also referred to as attenuating layers) for acoustic sensors utilized in downhole applications. Thus a need especially exists for an acoustic sensor having an improved transducer element, impedance matching layers, and backing layer specifically to address the challenging demands of downhole applications.